Not too close.
My most recent own example:
- lost a swarm
- they ignored my own trap within 10 meters (really meant for the incoming swarms; I'd be surprised if they took it).
- they landed into my other trap about a mile away.
To intercept outgoing swarms, I'd go for about a half-mile (+/-).
Realize, it takes just few flying seconds to cover this distance.
With such placement a trap will work better for all swarms (passing by and outgoing).