I did this before the commercial alarms were available. One of our Customers lost every condenser coil on their roof twice and this is what I came up with almost 4 years ago. Still operational.
Just a few of the 17 package units that are alarmed up.
Control wiring. So basically a 24v holding relay getting power from unit transformer wired in series with two loss of charger switches ( Y1 and Y2 ) on the liquid lines that will trigger the timer which is set up as a "one shot" timer to activate the siren for an hour. Also killing the disconnect will drop out the relay triggering the siren too.
Holding relay is DPDT so I use the other set of contacts to trigger the building silent alarm by running a two conductor from every unit in series above the grid. Any unit will trigger building alarm which is tied into the alarm panel as a dry contact.
Cutting some of the "obvious" power wires to the timer and to the horn will trigger the horn. Actual power wires for the Horn are well hidden.
Horn in the Heater section.....it's loud.
Another pic of the horn. Nice of them to vent the panel for me.
I know they have commercially available systems now but this was before their time.