Barking for Attention

Hi there - I'm new to this so bear with me if I post to the wrong section or what not.
We got a puppy about a month and a half ago and I've been lucky to have a boss that has allowed me to bring her to work with me. She's usually really good - just hangs out, sleeps while I work but in the past week or so she gets a little bossy and barks / howls (she's part hound) to get attention. Usually just after I put her back into her "bed" (crate) after a pee / walk or lunch time - after she finishes with her Kong or buscuit ball or whatever I gave her. Only for about 2 - 5 minutes before she settles down but I heard my boss asking my co workers if she's becoming a nuisance and I'm worried that he'll make me start leaving her at home if she keeps it up.
We do a 15 minute walk and 10 minute fetch in the morning and a 45 minute walk during lunch with another 10 minutes tug / fetch play time. (Plus another 45 minute walk after work.) Is this not enough to tire her out? She's a coonhound / border collie mix - just over 4 months now. She seems to get tuckered and fall right asleep as soon as she stops.
Any suggestions on how to teach her to be quiet on command?
I've tried the gently grabbing the muzzle and quietly telling her to "shhh" when she's loud. It works at home, but not while I'm at work. I've also tried banging on the back of the crate where she can't see me and draping a towel over the crate so she can't see me when she's loud. Not working.

Thanks for any input.

I do not have a dog, but I have read some interesting stuff about dogs barking on this board... one thread was pretty recent.
All I can remember was a collar that sprayed citronella (a scent dogs don't like?) when the dog barks, making it stop.
Or a noise type thing that plays a sound only a dog can hear when it barks.
But I was thinking a dog whistle? I don't know what kind of work you are in so I don't know if you can have a dog whistle hanging out of your mouth waiting for her to bark! :D Good luck.

Does your dog ever spend time alone? Maybe you should start leaving her for periods of time in her crate without you around. I'm not sure this would help the barking but I'd be afraid that she would get too used to my being there all the time.

What we had to do with our puppy was crate him, and leave the room for a while. When he would make noise, we would say "HUSH" in a firm voice. And he wouldn't get any attention until he stayed quiet for atleast 5 minutes without whinning or barking. It may or may not work for your pup. All learn differently.

Thanks all for the input / comments.

She's with me most of the time. Two or three times a week she's crated while we go grocery shopping or out for a few hours, but not very long - 5 hours has been the longest. I haven't done the spying thing, but I haven't heard her barking when we leave or come back.
I was / am worried that she's growing too attached to being with me all the time so I've been trying to take myself out of the picture when we're at home or at other peoples places (just locking her out of the room) and she seems fine.
Late yesterday and today - when she gets ready to bark - I've been putting a towel over the front of the crate so she can't see me anymore. So far it's worked! :) No barking today at work! (so she may have saved herself from banishment to the shop!)

Thanks again!