How can I find and remove the source of "full screen" ads when I can't locate it in any of my apps?

There is a site/app that takes up my whole screen (LG V30 Pie) several times a day. The top part, in the blue area, is always the same. The rest can be scrolled. None of my apps allow "draw over". The AdLock app does not catch them. I have no idea whether it's just an annoyance or dangerous. Short of resetting my phone and having to reload all my apps etc., can someone please help me find and get rid of this annoyance? NEW INFO: I have looked at the suggestions. Thanks. Andrew T. provided an idea but it uses functions that I don't know how to do, notably, the adb shell commands. Is there somewhere I can look to see how these are used, please? CANCEL: For some unknown reason, this problem has gone away. I'm not aware of what I might have done ... it just suddenly stopped a few days ago.