Theft was a problem in Roman bath houses, so slaves watched bathers' property and curse tablets punished those who stole.

The Long-Standing Problem of Thieves

As far back in Latin literature as the comedy writer Plautus (254-184 B.C.), thieves plagued the Roman baths. The early novelist Apuleius (c. A.D. 124-c. 184), satirizes bath thievery as the basest form of larceny. Catullus, Seneca, and Petronius also mention thieves who lurk about the baths.

Perhaps the best evidence of these thieves comes from the victims themselves: They called on the gods for help, by means of very durable curse tablets.

Bath Thieves' Victims

Today people talk about streamlining closets, donating items in the closet not worn for more than a year, discarding items that have moth holes, etc. That we can do that is evidence of our relative luxury. In ancient Rome, during the period from which we find curse tablets, there were not many who could afford to act as we do. It was a time when not everyone could afford a closet full of "outfits". As has been true through much of history, only the rich had lots of clothing. If your garment was of wool, you needed access to the back of sheep, and then it took time to spin and weave that sheep's wool into a piece of cloth suitable for covering one's body. So to find that Romans were attached to their garments was more than an issue of modesty.

When we think of bathing, we assume nudity, but whether that was true of the Romans is debatable.

While bathers were described as nudus this could mean scantily clad instead of naked. Whether partially or fully nude, however, bathers shed their clothes before entering the water.

At the baths there were storage nooks for clothing, utensils, oil and strigils (used almost as we might use liquid soap on a body scrubber), but we don't know whether these were fitted with doors or locks. And even if they were, the locks could be picked.

• Pompeiian Bath Photos - Storage Lockers
  Description of the method used to remember one's locker.

It was better to have someone looking out, so some bathers hired the bathhouse slaves (capsarii) to watch over their belongings. Others, wealthy, brought their own slaves. However, even a slave was no guarantee that the clothing would be there when the bather emerged. Hot as the baths were, it was easy to fall asleep. We know this from admonishments directed at the slaves to stay awake on account of bath thieves:

ne addormias propter fures.

Slaves could also be tempted by profit and sometimes sold their masters' garments.

Curse Tablets and Sympathetic Magic

Imagine emerging wet from a swimming pool to find your clothing gone. How would you get home? Would a kindly stranger lend you a jacket? Would you notify the police?

The Romans didn't have police, but they did have something else that worked as a deterrent, the fear of the gods.

A victim of theft might seek the god's vengeance or double the likelihood of divine help by transferring ownership of his stolen garment (or other article of value) to the god who would then want to retrieve the garment in his own interest. Generally this was done at places like Aquae Sulis (Bath, England) by means of curse tablets which were pieces of lead or pewter rolled or folded and thrown into the spring or nailed to the bathing establishment. Inscribing on his piece of lead the victim would call on the god to right the wrong, by bringing the criminal to justice and retrieving the lost article. Some curse spells were thought more binding -- especially the ones written backwards. One hundred thirty such tablets (not all directed at bath thieves) have been retrieved from the Sacred Spring at Aquae Sulis. [Read what the About.com Guide to the UK has written about the modern experience of Aquae Sulis in Bathing in Bath as the Romans Did.]

One typical victim asked Minerva to make the thief (whoever he might be) purchase the curse tablet with his own blood:

Minerv(a)e de(ae) Suli donavi furem qui caracallam meam involavit si ser(v)us si liber si baro si mulier hoc donum non redemat nessi sangu(i)n[e] suo.

In general, the tone of the tablets displayed at Bath seems excessive, out of proportion to the crime.
It seems the curse tablet must have had the power of the proverbial voodoo doll, for thieves, learning their names had been inscribed, were often frightened enough to purchase the curse tablets to secure release from the curse.

Sources:

Garrett G. Fagan: Bathing in Public in the Roman World. University of Michigan: 1999.
The Museum at Bath.

Online Resources

Bath Glossary
Resources on the Baths
Pompeii Bath Photos

Offsite
Early Greek and Roman Baths

Bath Terms Glossary

Citations from Garrett G. Fagan's Bathing in the Roman World
Plautus (Rud. 382-85)
Seneca (Ep. 56.2)
Catullus (33.1)
Petronius (Sat. 30.7-11)
Apuleius (Met. 4.8.9.21)