Hi Angelika,
We understand you're seeing a message about camera activity on your iPhone, and we'd like to help. To get started, can you tell us which iOS version is currently installed in Settings > General > About? Also, do you currently use any security cameras that may be linked to the Home app? If so, this message may be referring to those connected cameras. We have more information about setting them up here:
HomeKit camera security
Set up security cameras in Home on iPhone
If you aren't currently using any home security cameras with your iPhone, can you let us know if any other apps were open besides Messages and Calendar? We'll keep working from there.
Take care.