hi there,
if you are doing a regular office clean, and you have ops doing the jobs, instead of yourself, ask them to watch out for desk thinning.
another one is when the removal crates are in the premises, but no one has told the cleaning company they are moving, ive had clients in the past ring on a thursday, saying " we are moving over the weekend, last clean friday" they really dont care some of them.
also personnel changes, are a good sign, that something although innocent may be afoot.
regards
martin