I left one job to switch to another. A year later, my new job started using a cloud-based task management app. When I went to sign in, my 1Password auto-filled the credentials I'd used for the same app at my previous job, and there I was looking at all of my old employer's current projects and other confidential info. I called my old boss (who I got along with just fine), told him what happened, and asked him to please cut off my access immediately.

When you leave a job, it's in your own best interest to make sure that all of your access is removed. It's a lot harder for them to blame unexpected happenings on you if you can't even log into the thing. (Not that this happened here. I just wanted to point out a gotcha you might not have thought about.)

If you find out that they missed something, report it to them immediately and keep that paper trail demonstrating your good intentions toward them. Then hound them about it until they get around to fixing the situation. And for the love of God, don't ever, EVER log in "just to look around". Absolutely no good can come of that.