Help! Faint smell every now and again?

Prius and Escape transmissions are kind of at opposite ends of the spectrum.

A Prius transmission does not include any clutches, bands, drums, sheaves or link belts. No friction. The fluid is not pressurized or strained to move the car. It's just splashing around to lubricate the gears and cool the electromagnets. It sees waaaay less stress than the fluid in most other transmissions.

This means most people go forever without ever changing the fluid, and the fluid never gets boiled or burnt into the sort of crud that requires flushes to remove.

So... changing the fluid isn't a terrible idea, but you shouldn't even feel like that is a major necessity. You aren't burning your fluid, so you aren't smelling it. If you do decide to change the fluid, insist on real Toyota WS fluid. You don't want something to eat the insulation off the electric coils in your transmission so chemical compatibility is more important than usual.

A lot of words to say: It almost certainly isn't the transmission.

Now, you still have a smell to figure out.

I would start with the battery cooling fan under the left rear seat. You should get familiar with that anyway, as a part of regular maintenance. There are youtube videos that show how to get the panel off with a screwdriver, and then how to clean the fan and tubing system using a vacuum cleaner.

You can take a screwdriver down to the car wash and use their vac if your driveway situation isn't great. Doing that a couple of times a year is going to help you get maximum life out of your hybrid battery.

 

I had a 2012 Prius C, similar issue, intermittent electronic burning smell - usually in the mountains, when I'm working the traction battery really hard. Do Not place any bags behind the drivers seat. That round air intake will suck the bag against it and block cooling to your traction battery. In my case, I always threw my sunshade in the seat rear floorboard area. Every once in a while they would slide over, behind the drivers seat and plug that intake vent. You can verify that the smell is coming from your traction pack by placing your hand on that plastic covering below your rear seat bum cushions. It shouldn't be warm, because airflow from the traction battery cooler should be keeping it cool.

Hope this helps....

 

I'd recommend you try to find the general area of the car first, before asking asking to have something checked and not know where to have them look.
Next time you notice itl, find a place to pull over safely and put your sniffer to work, around the dash area and in the back near the floor vent , back seat and hatch / trunk.
If you can find where it is coming from, you'll be in a much better position to get some help with what the problem might be..