Actually arrival is pretty simple, tag gets into a range of receivers antenna, but the departure is what is causing the problems.

First some information about the setup we have.
Tags:
They work at 433Mhz, every 1.5 seconds they transmit a "heartbeat", on movement they go into a transmission burst mode which lasts for as long as they are moving. 
They transmit their ID, transmission sequence number(1 to 255, repeating over and over), for how long they have been in use, and input from motion sensor, if any. **We have no control over them whatsoever.** They will continue doing what they do until their battery dies. And they are sealed shut.

Receiver forwards all that data + signal strength of a tag to our software. Software can work with several receivers. Currently we are using omnidirectional antennas.

**How can we be sure that the tag has departed from premises?**

Problems:

* Sometimes two or more tags transmit "heartbeat" at the same time and no signal is received. With number of tags increasing these collisions happen more often, this problem is solved by tags randomly changing their heartbeat rate (in several milliseconds) to avoid collisions. Problem is I can't rely on tags not "checking in" for a certain period of time as sign of departure. It could be timeout because of collisions. Because of these collisions we cannot rely that every "heartbeat" will be received.
* Tag manufacturer advised that we use two receivers and set them up as a gate for tags to pass through. Based on the order of tags passing through "gates" we can tell in which direction they are going. The problem with our omnidirectional antennas is that sometimes tag signal bounces of building and then arrives to receiver. So based on signal strength it looks like its farther away then it is.

Does anybody have a solution of what we can do to have a reliable way of determining if tags are coming or leaving? Also we can setup antennas in different way as well.

I wrote the software that interprets data from receivers, so that part can be manipulated in any way. But I'm out of ideas of how to interpret information to get reliability we need.

Right now the only idea is to try out with directional antennas? But I would like to tryout all the options with the current equipment we have.

Also any literature suggestion that deals with **active** RFID tags is more than welcome, most of books I've found deal with passive tag solutions.