- AirTags send out a ping at random intervals. using a randomized and changing ID number. The interval between pings changes, it takes into account how long it's been since it's seen owner's phone, how long since it's seen any phone, and if the tag is moving (it can sense that its bouncing around, not that it's actually traveling)
- Those pings are picked up by an apple Device (iPhone, iPad, or Mac**)
- the device that hears the ping sends the randomized ID and the location of the receiving device. If the receiving device doesn't have a network connection, this can cause location errors. I've seen people here saying, where in spotty cell service, tags were reported a couple hundred yards down the road where there was a small pocket of cell coverage. The receivers were driving, so the time between ping received and location update was pretty short, but the distance was decent.
it's possible that it has something to do with that train track, and someone traveling along that. But seeing as you appear to be in London, cell coverage should be fairly solid.
tags also don't consistently update, so just because it says you were there for 10 minutes, it could just mean that it updated while you were driving through there, and then didn't update till 10 minutes later.
Also, from reports here, it seems that tag sharing is fairly laggy, so if you're sharing the tag with someone, they very likely will have old / not frequently updated location info.
**Macs don't have GPS location, and will use the wifi networks around to guesstimate where there are, It's possible that someone moved a base station to a new location, which is causing confusion.