[RPG] Can a caster tell when there’s a thin sheet of lead blocking detect magic

I did some research and cannot find any sort of official ruling for this. Although it seems to me the the answer is clear.

You can whisper messages and receive whispered replies. Those nearby can hear these messages with a DC 25 Perception check. You point your finger at each creature you want to receive the message. When you whisper, the whispered message is audible to all targeted creatures within range. Magical silence, 1 foot of stone, 1 inch of common metal (or a thin sheet of lead), or 3 feet of wood or dirt blocks the spell. The message does not have to travel in a straight line. It can circumvent a barrier if there is an open path between you and the subject, and the path's entire length lies within the spell's range. The creatures that receive the message can whisper a reply that you hear. The spell transmits sound, not meaning; it doesn't transcend language barriers. To speak a message, you must mouth the words and whisper.

By RAW, it specifically says the in the spell, you must mouth the words and whisper.

As for how you should rule? What makes sense for your campaign? Vote, houserule, or go by what the spell says.

First, who has ever actually encountered such an artificially contrived condition in actual play?

Second, if the target is within range and there is an unblocked path of any length the spell works. The range of a spell is a sphere around the caster. Normally a spell requires a clear line of effect in a straight line between the caster and the target but the specifics Message override this by stating "The spell doesn't have to follow a straight line and can travel freely around corners or through openings." It doesn't put any limitation on how long this path can be.