The last company I worked for had to sack someone for gross-misconduct. He had a company laptop at home and also the office key.
When he caught wind of the fact his game was up he wrote an aggressive email threatening the company (it was a company of 5 people) and the MD with unfair dismissal and all sorts of other things. The MD wasn't having any of it, and demanded the laptop and office key be returned. The (ex)employee said he was keeping them in lieu of the wages he was owed and the several months he was demanding on top of that for his unfair dismissal.
The MD got the police involved (for theft of the company property), and they were reluctant to do anything at first, stating it was a civil matter as the ownership of the property was in dispute. However, something else must have interested them about the employee, as they arrested him in the middle of the night, and confiscated the laptop and the keys. They were obviously interested in the computer for whatever reason and what was on it, and so they told the MD that they'd be keeping hold of it for their investigations. 5 months later and he still hadn't got it back! I'd since left the company to go self-employed, but the last I heard was that the company still hadn't got the laptop back and that was at least 9 months from when it all started.
In hindsight I think it's best to make sure that any expensive equipment is signed for by the employee on a document stating that it's the company's property and must be returned on demand, etc.
Best of luck with it - doesn't sound like a very pleasant situation.
Poorly disguised "I got sacked for having porn on my laptop"