I'm going to guess, if she was truly pregnant, and she had been drinking, that would instigate an argument. If she left angry, and possibly a little tipsy, or even flat out drunk, then erratic driving would be expected. That car could be in a retention pond, a canal or lake nearby. Drones would be helpful as would Google Earth for that matter.
I'm curious to know why they suspect foul play - based on the wallet found in a library? Maybe but it could have washed up from a body of water, laid on an embankment and been picked up by a kid. The rainy season combined with flooding dredges up weird stuff all the time.
"Police say Teresa got into an argument with her boyfriend after returning from a bar. She drove off in her 1966 Nash Rambler........"