Why?
She probably used it as an antacid. Baking soda works well mixed with a little water an injested.
Or to brush her teeth. Or as a natural deodorant. My sister uses a combination of baking soda and cornstarch in place of Shower to Shower or baby powder. There are a lot of uses for baking soda beyond baking.
:: furrows his brows in confusion ::
Why? What do you get out of it, besides filling up your suitcase, chancing a spill, and potentially getting in trouble? Is this, like, really, really expensive detergent or something?
If you do take it, though, I recommend tying it in a baggie and swallowing it. 
Well, I’ve been doing some research-by-Google, with the following results:
Travel-related sites including “laundry detergent” on the list of things to remember to pack: lots
Sites mentioning a security problem with bringing powdered detergent: 0.
So between that and the advice (above) of an actual airport security guy, 3.885AM, it looks to me like I was just being paranoid.
They’ll probably confiscate your travel stash anyway, so you may as well just buy some when you get to your destination. It’s not like the Costco box is going to go to waste.
Also, IME the security guys in European airports don’t search you as thoroughly as they do in American airports these days.
Magellan’s website and Travelsmith’s website probably both have small sealed packets of detergent designed especially for travel for sale. Also, fairly small boxes of detergent are available at many stores and certainly at laundromats.
Why not just buy it there though?
What Zoe said.
I’d go down to the local laundromat, and buy a couple of single size boxes of detergent. Dryer sheets too, if you use them. That way, the boxes are clearly labeled and are sealed too. When (if) security looks through your bags, it won’t cause any problems.
That couldn’t possibly cost more than a couple of bucks. Those boxes are only $.50 here.
Why not just go to the store and buy a small bottle of liquid detergent. Put it in your bag and go to the airport without any problems.
Brought the detergent in my backpack through security at Dublin and Heathrow Airports without so much as a second glance.
Sorry I got here late. I do this all the time when I travel. You need a bag to separate soiled clothes from clean, so I chuck a cupfull of detergent powder in a leaf litter bag. I then use that bag as my laundry bag (the detergent also keeps the smell down in small hotel rooms). When wash day arrives I just go to a laundromat and dump it all in. If I’m going long enough for two loads I keep the second lot of powder in a lunch bag and now add it to the empty garbage bag. Never been asked even if luggage was searched (that has happened twice - my turn I guess).
Ha. So the trial run went ok? 
Suggestion?
Go to your nearest laundromat and buy one (or more) of the single-size packets of detergent from the vending machine. This way, when and if some agent runs across it, at least it looks more innocuous that some white powder in a baggie.
I’ve never actually seen any detergent vending machines in Ireland, but thanks for the suggestion.
Hmmm…do I smell a business venture begging to be exploited?..CHA-CHING!!!
Haven’t heard back from ruadh about the detergent caper. Suppose he’s in Gitmo?