I believe they should only share sensors when they are in your squadron or another pilot is in the cockpit (and assigned I think). Might need to have IFFs being friendly towards each other too. Sensor sharing has been a bit wierd with regards to how it works exactly, but just empty ships will definetely not share.
@Joy: Of note, even if you Do have increased Vision, you can still only "scan" within your ship's range.
@Mesh: I wouldn't think so. "Unmanned" aka not having the pilot in the cockpit don't share sensors and aren't supposed to. I'm pretty sure the squadded ships share sensors with the lead ship when the lead ship is manned, though.