When we transitioned my son from his crib to the toddler bed, he had a habit of getting up and wandering. He would come into our room and wake me just to say he loved me. He was a little over three at the time. I read in Parenting magazine about this, and they suggested when you put the child in his "big boy" bed, to tell him you will be checking on him in a few minutes, then leave. Go back in his roon after a few minutes so he knows you're still there. Continue until he falls asleep. Then increase the time between your visits, 3 minutes, then 5 minutes, then 10 and so on. If he get's up in the middle of the night, he probably just wants to make sure you're still there. Gently, walk him back to his room. I also set up the baby monitor and showed my son that I could hear him in his room if he needed me. It took about 2 weeks, but the wandering stopped. Hope this helps.
Take care and God bless! J.