As established in Positronic signature in Nemesis, positronic signatures are detectable at a considerable distance by Starfleet equipment, and Starfleet vessels seem to regularly scan for such signatures. In particular, it was decided that positrons are likely to show up even on passive scans of nearby space. (The motivation was that, in Nemesis, the Enterprise detects B-4's positronic signature from several sectors away.)
In the episodes "Time's Arrow" (Parts 1 and 2), Data's head is knocked off in a catacomb below San Francisco in the 19th Century, and the head spends several hundred years there until it is discovered during an archaeological dig in 2368. This sets in motion a series of events that includes the Enterprise coming back to Earth to investigate and several members of the Enterprise crew travelling to the 19th Century, which in turn completes the loop of Data's head being knocked off. (They also manage to bring Mark Twain to the 24th Century.)
My question is:
Why wasn't Data's head detectable to Starfleet equipment and other scientific equipment on Earth, or to Starfleet vessels in orbit around Earth?
Comparing to Nemesis, B-4 was in pieces on the surface of Kolarus III and certainly not in any kind of functional state until he was reassembled on the Enterprise. Yet, he was giving off a steady positronic signature.
Was Data's head not emitting a steady signature? If not, then why? If he was emitting one, then why wouldn't the myriad of Starfleet equipment or vessels near Earth have detected the head well before a group of archaeologists working by hand?