Snow and fog (Dew) are both good indicators
Regarding Metal Detectors.. Instead of spending money (Till you find out if it works) did you knwo.. There is an APP for that????
Yes. many Smart phones do much more than talk, text, camera and internet.
They have a host of "Sensors" including
GPS (Where are you)
Magnetic (Compass)
Pressure (Barometer)
And one that detects the presence of metal
I have a "Metal Detector" app on my phone.. Free.
I have not had great success using it, but I have had some success.