Is this a formal spoken or informal spoken ?

That sentence does not make clear what are you trying to say, Arafat.

Are you actually talking about "what you can always see?" (I doubt that.)

If you are trying to say, "Sometimes people use detecting devices to find stuff lost on the beach by tourists"...just say that.

"I often see people using metal detectors trying to find lost stuff of tourists hidden underneath the sand."

Ooops..allow me to correct my previously suggested sentence.
It should read: "I often see people using metal detectors trying to find lost stuff hidden underneath the sand which belongs to tourists ."

Otherwise, this sentence would sound as though it was referring to the tourist hidden underneath the sand. This grammatical mistake is known as a misplaced modifier - which is something that can catch us native-English speakers unaware.

Thx for all ur passion to my question