2.5 year old night waking

My son is 2 1/2 years old and has traditionally been a good sleeper. however he has started waking in the night, between 1 & 2 times, and i cant figure out why. He goes to bed about 7.45pm and is placed awake in his own bed. He settles himself quickly and we often dont here a peep from him. Recently he has started waking (at various times of the night) and comes into our room. I then take him back into his room, tuck him back in to his bed and leave. He then settles himself quickly. He isnt wet, doesnt ask for a drink or anything and doesnt seem visibly upset from a dream. Some nights this happens once, other times it can be three times during the night. I thought he might be cold and have started turning his heater on as the nights are now getting chilly but this hasnt made a difference. While it is easy to get him back to bed the broken sleep is starting to effect me as i am 17 weeks pregnant and often have trouble getting back to sleep once he wakes the first time.
Any thoughts and ideas would be appreciated.

Do keep up exactly what you are doing and let him know it’s time to sleep and go.
It may take a couple of weeks but you are definitely doing the right thing.
Keep up the good work.
Cheers Maree