Once a day or two, a small icon button appears on the side of the screen. Got a virus on android?

Question

Android

Periodically on the main screen, after unlocking (about once a day), a mini-button-icon appears (each time different) stylized as a bookmark, when it is pressed, a window opens (not full screen) with advertising.
viewing the list of running applications in the settings does not show any suspicious applications (but there is no identifier, only the name is visible).
Attention to the question, how to catch the application as simply as possible? what taskmanager to catch it?
root is not and is not expected (not my phone).
Xiaomi Redmi 3 pro
miui 7.3 global stable 7.3.2.0.0
has very few third-party applications installed on the smartphone (no more than 20), but I don’t want to remove one at a time and see what happens, as it takes a lot of time.

Answer 1

есть партнёрские программы, которые занимаются тем, что если приложение скачано не со стора, то начинаются подобные странные штуки. А если со стора, то всё ок.