I have cited the source which printed this story in the German language (AMuS). Michael Schmidt is a very reliable source in F1.
Webber changed from tub 02 to tub 04 for Spain and Vettel continued to use chassis 03 which must have been tested back from China for the first time because during the fly away races they had no opportunity to ultrasound the chassis in the factory. Presumably the crack wasn't very severe at that time that it escaped the test or they only did a visual inspection because at this time Seb did not have any complaints.
In Spain Seb started to go slow with tub 03 but they had a back to back race in Monaco and that time they could not check the chassis. Only after Monaco they did a very thorough check in the factory and found the crack in the front lower wishbone pivot point. They decided to give Vettel the 02 chassis which Webber had discarded as no other new chassis was available. It appears that he will use this in Canada as well. I hope Red Bull will quickly prepare a reserve.
Obviously the issue with the suspension geometry would only become apparent when the wishbone pivot point would move under load. So a normal suspension check would not reveal that problem. Sebastian told the press that with the new chassis he could suddenly feel his car properly again and position it into the corners. I guess if you cannot do this you loose all your confidence and yours speed.
I am confident that from Canada Seb will be the faster Red Bull driver again if no more defects affect him like in Turkey.