Oh. I didn't actually pay attention ... Sorry
Bakwena use also etags as what Havoc said, and if I drive under the GIFP gates it beep 3 times, to say the gantry and my tag talks, but isn't registered, so no money taken, also some gates like the one before the Hugenote Tunnel, beeps, but no money taken.
When It beeps once, then they steal your money!