DS gets up once or twice a night. Generally he comes into our room looking for "snuggles", but he has also been known to go to the spare room and make himself comfy in there (regardless of whether or not there's anyone staying in there). He usually goes straight back to bed with no fuss and goes back to sleep, so I wouldn't get too bothered by it except for the fact that it's screwing my sleep totally leaving me exhausted and he's really tired and therefore horribly grumpy in the day.
There's been a lot of change in the family recently - I've just started back at work after being off sick for nearly a year, for a start - so I'm thinking he might just be feeling a bit insecure and this will pass. But this has been going on for a long time so any suggestions as to what might be disturbing him or what I can do differently would be welcome.