Has anybody ever heard of an employer paying full wages while going on Active duty for an indefinite period of time?

It's usually the differenve in your wages, not full wages. It's offered to people who are activated for duty, not those who join active duty. For instance, I knew someone who was a Paralegal in Washington DC. She was activated as part of a COAD. She was an E5 and took a massive pay cut. Her old company paid her until she decided to take a full time civilian position overseas

Ma'am,
Even if it's partial wages I suggest putting them in for an ESGR employer award. My civilian job pays me for the first 120 hours of work I miss each year. I've put them in for an award every year for that and other reasons. The USERRA only requires time off for military duty; pay for time not worked is not required by law so any amount is above and beyond.

I knew a guy whose company offered him the difference not including BAH. Though he worked for a nuclear power plant and was National Guard.

Depends on the company. But I worked for AstraZeneca pharmaceutical in Texas and they paid my full salary, which included bonus for a year when I was on orders for 365 days. However, it was only a one time thing.

That sounds very generous. This benefit should be described in the Employee Handbook (or similar document). Be sure to read the fine print. Good deals like this are hard to come by! Sometimes they are limited to involuntary mobilization, for example. I worked for a private company that paid the difference between military pay and the employee's salary while they were mobilized. The Government has rules about reserve duty and mobilization explained on the OPM web page. If I recall correctly, civil servants who are reserve component personnel are allowed 160 hours of military duty pay per year. This number may be increased to 320 when the employee is involuntarily mobilized.

When I first entered as a reservist my employer had an arrangement such as this. Basically if you were a reservist or NG or were called upon for jury duty they would pay you as if you were coming to work each day. The thing was this. The pay was not on top of your military or jury duty but rather it was the difference between what you earned while on duty and what you would have earned in working for the company.

Granted that at the time the US had not had a mass call up since Vietnam. So the most they had to really worry about was one weekend a month and the two weeks each year and the occasional actual trial.

That all depends upon the individual employer, but usually most will pay you the difference between your private sector job salary and the pay you receive while on AD orders. As for me, I paid my one Reservist employee his full income while deployed or at AT and various trainings because these are residual commissions and it is his personal book of business.

I was told at one time Amazon would pay full salary while on orders.

If you work for a government agency - federal, state or local, they will have legal requirements. Many private employers have their own policy, but many do not. All have to let you go and take you back without discriminating against you. It's always suggested you keep them informed as much and as soon as possible and get in writing from them what their policy is on how much, if any, they will pay you while on orders. In my federal and private sector jobs, they've fortunately paid up to a month after I was on orders. My current job is even more flexible than that.

As said already, not indefinite but for example: Las Vegas Metro PD will pay full wages if you go on active orders or deploy.