Occasional smell coming from the A/C vents

Hello,

Engine coolant does have a distinct 'sweet' smell to it.
It is possible to have a small leak in the heater core that is just enough seepage to make the noise and not enough to be noticeable immediately on the coolant level.
It will be a good idea to inspect the heater core for any leaks.

https://www.2carpros.com/articles/replace-heater-core

You can also do a pressure test on the cooling system. On the pressure test the cooling system gets pressurized with a tool, and if there is a leak; the system will loose pressure and that will show up on the gauge.

Here is a helpful video showing how to do the pressure test:

https://youtu.be/W8kra65m15c

I have attached diagrams for the heater core below.

Please let me know if you have any questions.
Thank you.