Hi,
Server can be disconnected by few reasons:
1. Connection has been closed on client or server side. OnDisconnect fires immediately.
2. Hardware has been removed on server side. OnDisconnect fires immediately,
3. Hardware has been removed on client side. OnDisconnect fires fast but with some delay that depends on BT HW implementation (when the hardware detects RF link lost).
4. Client become out of range. OnDisconnect fires with delay that depends on dongle implementation (it needs to detect link lost and reconnect attempt timed out).
It never decide to disconnect on the RSSI level. RSSI is something not usable in most cases (in some it is usable) because of some reasons. RSSI depends on dongles hardware, environment, power settings and lot of other things.