Isn't that exactly what a stakeout is? Or an undercover operation?

I didn't search for undercover, but I can find nothing about a warrant being required for a stakeout in the US (and numerous results saying no warrant is required).

Sure, and if the same level of effort were required to get the same amount of information, this probably wouldn’t even be up for discussion. But now we have technology to basically stake out every single citizen at any time with just a few mouse clicks. We can’t just pretend those things are equal.