How do I get my sim's motorcycle back?

Id honestly probably just cheat myself the exact amount needed with familyfunds and then buy another one. Im sure theres a mod way, but I have no idea what command it would be.

If you have NRaas MasterController, you can click on the suspect roommate, NRaas > MC > Basic > Transfer From and select your own sim as "destination." Then you'll get a list of items in the other sim's personal inventory and can pick the motorcycle. You'll have to click on each sim separately—I don't know of a way to search all of their inventories at once.

It's also possible that your sim left the bike somewhere else. If you click on the ground (not on a lot or the mod will only search that lot), NRaas > MC > Object Info > Not Inventory, you'll get a list of all objects out in the world, including cars that are in use or stuck on the road somewhere. But it's also possible that Overwatch or Traffic deleted the motorcycle at some point, especially if someone else was using it, in which case you'll just have to buy your sim a new one.

I have the issue with my sims leaving their vehicles in parking spaces in town, if they walked or ran from the original community lot to others, then went home without needing their vehicle. I always look for it where my sim went first, and drag it into their inventory, the place it on the parking spot at home.

A day or two ago, my sims' speedboat was left in the water while they were snorkeling, without be transferred to their personal inventory as usually happens. When they went home, they took a water taxi instead of the boat. I couldn't pull it into their personal inventory, and resetting the boat trailer in their yard didn't bring it back. So I went back to where the boat was in the water, and reset it. That worked.

If you know where your sim went the last time they used the motorcycle, you might find it there in a parking lot. If so, you should be able to either pull it into your sim's inventory, or reset it with NRaas Master Controller, which would put it back in the parking spot at their dorm or house. If one of the roommates has it, what @puzzlezaddict said will work, but you can select all of the roommates at once by clicking on the map tag of the dorm or house to open the mod. I don't have my game open, so can't look at the steps, but I think it would be NRaas > Master Controller > Sim > Transfer From, then in the panel to select which sim, select all except your own sim, then select your sim as the destination.

If you don't want to look at their personal inventories with MC, you could save your game then switch to each of her roommates and check their inventories, then quit without saving and reopen your game. I used to check on inactive sims that way a lot, until I started using mods.

You might need to get Master Controller to be able to transfer it to your sim's inventory and remove all parking spots as those should've never been left in place for the sim to use to park that vehicle and I'd just buy a cheap bike for her to use for now.I wished I'd had my CC collections fully ready for uploading as it might've allowed you to solve that issue with a cheap scooter or a cloned bike costing next to nothing.You should be able to clone an object and reprice it with S3OC and use that non-default clone to get the vehicle replaced with a cloned one.

I do have the NRaas mods, thank you very much for bringing this up. I just need to keep a close eye on to see who has it now. I did look at all the locations she was at, the parking lots are all empty.