I hate to say it, but in the hotel where I work (and I suspect other hotels as well), there have been issues with maintenance workers, front desk staff, bell-hops, even other maids, etc, poking their heads into checkouts in the mornings and grabbing any tips that have been left for housekeeping.

Management has even had to do a few 'sting' setups where they deliberately set a marked $5 bill out in plain sight in a checkout room and then sent certain suspected employees in to check something, just to confirm if they do, indeed, take tips meant for the maids. For this reason, I recommend leaving a tip somewhere that a good maid will find it, rather than leaving it in plain sight. I usually leave a tip in the desk drawer where the hotel provides free stationary, because I know the maid will need to check that area when she/he cleans, and it's unlikely that any one else casing the room quickly will find it there.