JustJared has posted a few images that show Mark Wahlberg and the Optimus Prime truck filming in Los Angeles on April 9. Last week there were rumors of reshoots but at the time it turned out to be Megan Fox for Teenage Mutant Ninja Turtles. As the pics show, that isn't the case here. The use of the modern truck combined with Wahlberg's intact clothes and no wounds suggests that whatever is being filmed is for a sequence shot for scenes early in the movie and set in the United States. TFW2005 thinks that it might be before the Optimus Prime vs. Galvatron sequences teased in the trailers. Re-shoots are not exactly cheap and normally only done in cases of necessity. It isn't to say it not common, just that a true justification has to exist for the expense. If it was just a one day shoot, my best guess is either they needed to capture additional footage for a CGI sequence or discovered that a story point wasn't clear enough from existing footage.