On field standby today until 1600 w/ LSR tomorrow. Once 1600 rolls around, do I go straight back to LSR, or am I released until 0001 tomorrow?

Right back on long call as soon as field standby is over.

How can you tell? Master schedule doesn't show anything after 1600-should I be looking somewhere else?

Straight back to LSR.

It's in the contract, section 20, that if unused you revert back to LC.

It is in 20-K-6-g relative specifically to short call assignments. I can't find any similar reference for field standby. Interestingly, I called the crew desk (risky, I know) and they verified I was released from the end of my field standby block until the beginning of my next long call assignment. (0001 tomorrow.) Not sure if this was a scheduling error, but I guess it's on the tapes for today, anyhow. If anyone can reference the contract for field standby reverting to long-call, I'd be interested.

"20-K-5-b A Long Call Reserve must be phone available twenty-four (24) hours a day on his Reserve days until given an assignment."

The concept is that rest doesn't come after an assignment but before largely due to FAR117. After completing any assignment it's back to LSR, with some exceptions, like they can assign you an FSB straight from a SC (20-K-3-c-(6))

Work elsewhere have a question..
After a flight you are immediately on the hook for long call? Isn't that a continuation of your FDP?

No. Long call is not a flight duty period. Long call provides 12 hours to show, which is legal rest plus some. However, it is duty and is covered separately under,

"Before beginning any reserve or flight duty period a flight crew member must be given at least 30 consecutive hours free from all duty in any 168 consecutive hour period."

"20-K-1-b During a period of reserve days, the Company may release, or schedule to release, a Reserve into an Off-Duty Period of predetermined length."

Gives them wide flexibility to actual release you from duty. Beneficial to commuters who may be able finish early. For the rest it is, as you say, risky. Two-way contact opens us up to fix whatever is on their screen when they answer the phone.