Just like sales and other businesses which involve travel, environmental consulting firms will have a good chunk of their staff out on fieldwork. And that fieldwork is generally erratic. At most of the places I've worked, there were a few really big projects, but most of the time, the field staff had 1- to 6-week stints all over the place.
At each place I've worked, the scheduling and sign-out procedures were two separate items - one involved the planning, and the other involved, well, if you were in or out.
One time, I got in a wee spot of trouble because I left to do some minor, verbally agreed-upon fieldwork for a manager who was out on vacation. I didn't have management access to adjust the master planning schedule, but I did sign out. Some upper-level management folks went looking for me, and they checked the planning schedule and asked around to all the other managers they could find, and I was AWOL. After about an hour, they found my personal cell phone number by asking around (this was when I was still keeping my cell phone a semi-secret) and asked where I was. My answer? "Why didn't you check the sign-out book?"
Ok, well, that was a long time ago. Now, we have a fairly common scheduling software that you can use to see everyone's schedule. And we have smartphones and e-mail auto responses in case we are somehow out of range.
However, all that technology doesn't help if you don't update the schedule...or check someone else's. Just ask the guy who was trying to find me by calling my office phone repeatedly while I was out of cell phone range on a multi-week field project.