Unusual iMessage typing indicator behavior

Hello. I’ve been suspecting a family member on my find my has been using a secondary device as their location and or messaging through other devices, and I noticed something strange about their ellipsis in the contact list while they were typing. The dots slightly bounced as usual, but the actual ellipsis moved up and down as well in a way that no other contact’a ellipsis had moved before. I read somewhere that this could be caused by 3rd party apps mimicking iMessenger, or that the contact logged their iCloud into a computer and is sending the messages from there. Any truth to that?