The IRS gets copies of your 1099s from different brokerages so they can see the dates of sales if you get audited.

I ran into this in an extreme example because I sold almost all of a stock on bad news in one broker but had a little money with another broker so I bought the dump after hours on the same day and sold again for a little profit on the bounce. It made me wonder if they would go to the length of looking at the prices I bought and sold to try to determine when exactly during the day I sold and bought and if it violated wash sale rules. It didn't matter in the end because I wound up dumping it all but I wondered if I could get busted for it.