Everything you said was good except this, I think. The reason to try a screenshot is because if it shows up there, then it's obviously not really burn-in (of any kind... retention/AMOLED/IPS debate aside).
Now, if it doesn't show up in a screenshot, the OP should find a decent digital camera somewhere and take a picture of the screen and post it here so everyone can see what LCD burn-in looks like. ;-)