Years ago at a church I attended we had our own version of Bernie Madoff.
A very charming, friendly person joined the church. He quickly become involved in various committees and volunteered to do things like work the yearly Lady's dinner where the men made a nice dinner for the ladies of the church. He treated everybody well. Occasionally, he would drop a comment that indicated he was 'well connected' but he never boasted about it. He had a big house in the hills, drove two nice cars, his family vacationed in fabulous places. Again, he never bragged about it. It just came up in conversation.
Sure enough several people at the church got involved with him in a business venture. It was some sort of deal where the company, using new methods, would redo your bathroom - new tile, new fixtures, new cabinets, refinish tubs and showers etc. in only a few days for a remarkably low price. Needless to say all those who 'invested' in his great business venture never saw their money again. The last I heard he got divorced (apparently he conned his beautiful trophy wife also) and disappeared. The big house and cars were leased. The fancy vacations and the other trappings of wealth where put on credit cards that were never paid off.
My red flag is charming, friendly, helpful, subtle big talkers who I have known less than five years.