I'm 90% sure as well that that's MDC1200. Rather than tones, it uses a short data burst at 1200 bps. In your agency's case, this is likely used to directly cause the radio receiving it (sounds like either 2 or 3 radios in Medic 15) to make noise (beeps/tones/alerts) themselves. Because those alert tones would be generated by the end user's radio(s), you won't hear them.
Explained a little more in-depth: let's say there are 2 radios issued to Medic 15, one for each paramedic. Each radio has its own unique ID, generally 4 hexadecimal digits long (0-9 and A-F), i.e. 1234 or 9A7C or any such combination. The MDC1200 alerting, instead of a long pair of tones (minimum 4 seconds long per pair, usually), uses a very short data burst. Instead of having to listen for the full set of audio tones, the radio is only listening for its MDC1200 unique ID. When it hears it, it knows to beep or chirp or sound a siren or make some other attention-getting signal to the user.
The added benefit of MDC1200 in many systems is that you can also use it as a radio identifier when the radio is transmitting. Either before or after a unit in the field talks, do you hear a similar data "squawk" as the one you're hearing when alerting is happening? If you do, that means they are using this feature as well, which is known as PTT ID (push to talk identification).
Currently, no scanners support decoding MDC1200 data bursts, and the speculation is because nobody has really clamored for the support to be added - because the ability to decode it is out there. If you search Google for a program called "WinMDCd", you will find a small application you can run on your computer. With an audio cable plugged in to your scanner and the line-in jack on your computer, you can decode the MDC1200 bursts and display the IDs as they come across the radio channel.