Is a really low bid always a warning sign?

When you get low bid compare to other reputable companies, you should understand there is something wrong, could be that you need to go to bids and look on them see that it’s same scope of work, if it is the same DO NOT HIRE THE LOW BID. Sometimes it’s a way to step in the and sign contract then you start getting change orders and endup pay more then then high bid originally you could hire.

Yes it is. Because either the company is new or completely un professional staff
as well as the low bid signals for their bad works and behavior maybe they run from there jobs

It depends on how low. If the low bid is more than one standard deviation away from the median price for the service, that is usually going to be an issue - some companies may just run an efficient business and have certain advantages that enable them to offer lower rates. 10% below the median isn’t something to dismiss - but if the bid is coming in at like 30-40% below market value, that is usually going to be an indicator that the professional isn’t bidding properly and the client should expect to get hit with change orders.

I would 90% of the time it is a warning sign. Especially if ti is very low. Some companies have lower overhead such as myself and my bids typically come in lower. Some companies have larger overheads and their bids will be higher.