Could my enterprise proxy know the content of an https request?

Absolutely. Several enterprise-level proxies support re-encrypting the connections your browser makes using a corporate certification authority. Essentially the administration team can push out a certificate to your workstation via group policies, and add it to the list of trusted authorities. The proxy then has the private key corresponding to that certificate and generates a certificate for each hostname on the fly. Then when your browser connects the proxy uses HTTPS to connect to the destination, but then encrypts the actual tunnel to your browser using the aforementioned certificate and private key.

There's also open source and free proxies capable of this interception (which is just an MITM attack made easy by the administrators having access to the trusted certificate list on each workstation).

Edit: You can detect this by inspecting who has signed the certificate for each HTTPS site, but the name can even match existing certificates so you'd have to compare the fingerprint to a known good one of each certificate authority.