Random paper pieces security system:
When I feel the need (varies) I place all material that I wish not to be accessed either in a cupboard or a pile in a corner, place a blanket or similar over it all, tuck in the edges, then tear a sheet of paper in random pieces and scatter them over the heap and take a photo. I frame the photo in such a way that I can replicate it subsequently. On return to my room I take another photo and compare the two. Care needs to be taken that drafts from air conditioning, windows or opening a door will not disturb the paper.
The odds of anyone being able to disturb and replace all the paper pieces in their original position is essentially zero except if they take a photo and then carefully remove pieces systematically and restore them using the photo. ie a professional 'spy' or dedicated thief may be able to do it but the odds are minimal.
Updated: I just came across this example in my photos. Not as orderly as sometimes - and a confident person could risk feeling with their hand at bottom right - but it would be a risk and if they disturbed it there is no undoing it. And even the wrinkles in the blanket cover are part of the image available for checking.
I have used the same method once where I had reason to doubt the security of a locker. In that case I meandered a long piece of twine over the top of goods in the locker.
Twine in locker. Purpose obvious enough to the discerning. Very quick and basic but, without a camera no way to put it back accurately enough. Even taking a photo would make replacement extremely challenging. Note that camera position and framing is important for ease of comparison.
In no case has my paper or twine ever appeared to have been disturbed.
I sometimes use a version of the hair tell-tale. I place something either on the floor or against a door in such a way that it must be moved when the door is opened. The super observant intruder could look for such things but it's unlikely.
Motion triggered camera:
I have not (yet) tried this but it is among the more useful methods. Some cameras offer the ability to trigger on motion and to take video and/or stills according to a program.
While the cost of a new camera that does this may be high, there is a low cost and highly effective way to do it.
The free Canon SDK software allows many Canon A series cameras to have this facility.
Triggering can be on motion in a selected area of the image.
Suitable used cameras can be obtained at low cost (from about $10 up in NZ) and programmed for this and many other purposes. (I used one to do 10 second time lapse journey footage and sunset footage in Australia with great success.) For longer periods you may need an external battery or a mains power supply. (A small cheap sealed lead acid 6V battery would work in many cases.
Bonus: If the camera vanishes you know that there has been an intruder.
Sometimes I really really do not want cleaning staff to access my room or I want them to only access the bathroom. Usually this is because I have an immense mess in the room with papers and/or equipment arranged to suit me and do not want them disturbed or damaged. In such cases I have on occasion strung a string across the room at the "no intrusion" boundary and hung one or several relevant signs on it. These may not say any more than the sign on the door says but vividly convey how serious I am. It seems to work. It's useful to be able to get the bathroom cleaned without the rest of the room being touched.
On occasions I have had 'helpful' cleaners take material that was NOT in a bin or similar and that was NOT rubbish. If there is a risk of this and it matter suitable care needs to be taken.