First off, if the Sewer Contractor is working within the allowed Easement, then the only reason they'd notify you is out of courtesy. Beyond that, they'd HAVE to notify you either because they need to 'use' MORE than the allowed easement or because what they do may/will 'affect' your property.
Sure, I'd take pictures. And if you're out-of-town, I'd have a family member or friend take pictures. At the same time, if you don't know what's going on, pay a visit to your Sewer Office for information. Again, they (either the Office folks or the Contractor) will only tell you what you 'need' to know or is 'appropriate' for you to know. It depends on the situation. It's OK to be 'alarmed' at all the activity. And yes, I'd ctc. your Insurance Company. It's quite possible your sinking garage is happening as a result of the underground sewer problem. You MAY be able to sort of STOP the construction until you get more information. This is where you need to talk to the Sewer Authority Solicitor. AND, talk to the Sewer Authority Engineer too.
Assuming the damage to the garage is the result of the sewer problem, I wish you well on collecting your damages. (Talk to your Insurance Company.) You should get properly compensated. My best to ya and hope this helps.