As far as I can tell, Flights doesn't "officially" support flights from Switzerland yet. If you try to select ZRH as your only departure airport, you'll get an error message stating as much.
If you were recommended flying from MLH (aka BSL), I'd wager it's just the closest (active) airport to your geolocation. If you were suggested STR, your geolocation may be off by a bit.
To get some results for flights out of ZRH, select MLH as your departure, then, on the map, click the marker at MLH. A list of close airports (BSL, BRN) will pop up. Click "more airpots" and add ZRH to your selection. It then somewhat works. Take note of the list of unsupported airlines at the bottom of the map.
If you were recommended flying from MLH (aka BSL), I'd wager it's just the closest (active) airport to your geolocation. If you were suggested STR, your geolocation may be off by a bit.
To get some results for flights out of ZRH, select MLH as your departure, then, on the map, click the marker at MLH. A list of close airports (BSL, BRN) will pop up. Click "more airpots" and add ZRH to your selection. It then somewhat works. Take note of the list of unsupported airlines at the bottom of the map.