Revealing mistake: When Goldie Hawn pushes Kurt Russell off the boat you can see he has flippers on in the water.
Continuity mistake: After Dean discovers Joanna on the couch, her face covered in calamine lotion, he picks her up and carries her into the bedroom. As he sets her on the bed the viewer can see her face is free from any "spots" and/or lotion.
Other mistake: During the opening of the mini golf course you see in one shot that the ball comes out of Lincoln's mouth. Ever notice that there is no where for the ball to go? No artificial putting green is there, just a wooden platform and natural grass.
Continuity mistake: When Dean goes to the hospital, the police officer hands him a large manilla envelope containing the photos of Annie/Joanna. Dean folds the envelope in half, pressing it together firmly, then shoves it into a jacket pocket. Later, when Dean gives the photos to Billy, the envelope is flat and the pictures are in mint condition - no bends or wrinkles.
Continuity mistake: While Joanna and Grant are talking on the yacht while Dean is working on Joanna's closet, Grant has a bow and arrow in his hands. The first time we see the arrow it has a small blunt tip. The second time, the arrow has a big suction tip on it.
Continuity mistake: The school teacher visits the kids at their home and they 'toilet paper' her. One piece of toilet paper around her neck disappears and appears again when she's talking to Dean and then she drives off.
Continuity mistake: When Dean is picking up Annie at the hospital he goes through some photos. When he picks them up you see a picture of Annie. In the next shot on the table when he throws them down to look at her the first one on the stack was the second or third on the stack before.
Continuity mistake: When Annie goes to pick up the boys at school and they have poison oak, there is a shot of Annie holding her purse. Then they show her from behind with her purse over her shoulder, and then from the front again holding it.
Continuity mistake: When Grant and Joanna get in the limo, Grant is sitting on its right side. When they show the children running after the limo, Joanna is sitting on the right. They cut back and Grant stands up through the skylight, he's back on the right side.
Answer: This is just speculation, but since he knew what hospital she'd been in, he likely went there and asked who'd claimed her and where they lived. It wouldn't be too difficult.