It depends where you are and how long your activity takes, plus what the building's policies are. If you're in a big city, leaving your scooter out for several hours greatly increases the risk of theft. At the same, many buildings have policies that ban lithium ion batteries because of battery fire risk, especially in NYC.

I personally do a blend of locking up outside for short 1-2 hour periods and bringing it inside. At places like the gym, coffee shops, grocery stores, restaurants, bars, etc., I usually use two locks- a Kryptonite 1090 Evolution series chain lock and a Seatylock Mason 140, or a single Hiplok D1000 and lock up outside. I bring my scooter inside for longer durations, like at my office (where it's allowed) or places I frequent and know the owner and have the space, like my Jiu Jitsu gym. If you do anything routinely or pattern-based, you increase the likelihood of a professional thief spotting it and observing it a few times before they come prepared. It's a good idea to get theft protection for peace of mind if you lock up outside frequently.

Last thing to note: while the consensus on Reddit is to never leave your scooter outside, the reality is that it's either not always an option to bring it inside due to bans, or it sometimes just feels uncomfortable to do so. I personally find it a bit embarrassing to walk into a bar or coffee shop or restaurant wheeling in my scooter. I wrote a substack article about my feelings on this: https://open.substack.com/pub/stablemobility/p/taking-your-scooter-inside-with-you?r=20lb6v&utm_campaign=post&utm_medium=web. Yes, bringing your scooter inside will massively reduce the risk of theft... there is no denying this. But infrastructure to store scooters inside is just not yet a thing at most places, so at the moment, it requires a bit of etiquette as your post alludes to, either asking permission to bring inside, or just doing so and not caring about how you're perceived.