iPhone displays liquid detection alert despite not being wet

try a forced restart https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

After that get Apple Support https://support.apple.com/en-us/106932?choose-your-country-or-region=united-states

Sometimes condensation can trip the warning. Something like going from cold outside to warm room or car can cause condensation. Another common event is having the iPhone in the bathroom while you’re showering. Typically if it’s condensation, the warning goes away after a few days.