Yep, I think it's actually called a power ridge. It's in the aligner and it's actually an indent rather than a bubble where the aligner would fit over an attachment (so it's concaved rather than convexed). I didn't have one until my second tray, and now I have one on the top and two on the bottom. At first I thought it was a defect in the aligner and made a trip to see my ortho! It's not really visible at all (unlike the attachments).
Invisalign:
23/31 U (2/3 done!)
6/6 L refinements; Completed 15/15 L
Metal braces 10 years ago