My DS ALWAYS does this, now he is older though he does alot of crawling pracitce in his cot. I found that sometimes he would wriggle out when I left the room so I always checked him half hour after putting in bed and would put him back under the blankets if he had moved but...when he would wake, most of the time, he would be back up the other end or upside down so I assume he does it in the first few mins of being awake. I think they have a bit of play time then call you when they have had enough.
Although now, if I cant hear his head banging against the side of the cot then I know he hasn't moved. I think it gets worse as they get older.
Also, the sleeping bag things only work until they realise they can put their feet in the corners and push up. I have seen him dig his heels into the mattress and push himself up that way. There is no stopping the little escape artists