They Have A Profile On Tinder
This is a really bad sign. We all know what Tinder is for and it is not for meeting new friends; maybe friends with benefits. It is not as subtle as other signs but if it happens, know that either your partner cheated on you or is planning to cheat which is equally worse. In this situation, you can confront them and break up the relationship in time.
They can either tell you excuses, that they just made a profile on Tinder out of curiosity to see how it works. You should call their bluff and make them admit that they are cheating on you. Then the next logical step should be to put an end to the relationship once and for all and open a Tinder account yourself.