It happens, it does not make one irresponsible, maybe a bit too complacent, but not necessarily irresponsible.
I would argue that leaving a weapon such as a gun in an unsecured/uncontrolled location* and not knowing where that weapon is, is the epitome of being irresponsible.
*You may consider it safe while it was in his own home, but once he left that location and was out and about with no clue he was carrying a weapon - well that is a different story all together.
You may well be aware you have the weapon but not think about it when traveling. I have a pair of tactical boots that I keep a knife in. I never travel with those boots. Until the one time that I did. Thankfully I remembered when retying my shoe just before going to the airport. The knife sheath on that boot is such that it probably would have made it through the scanner just fine - it would have just looked like the ankle support on the boot.
While I don't carry that knife for self defense, it could still be dangerous to a child. I would still need to keep those boots in a safe location (or remove the knife) prior to babysitting a friend's child, for instance. Am I an irresponsible knife owner because I'm so accustomed to having that knife on me that I don't think about it? No, I wouldn't think so. Of course a gun is more dangerous but it could very well be that the lawyer keeps the gun properly stored in a safe and, as a ritual habit, put it into that laptop bag when he left for the day because that is always what he does when carries that bag. People fall into routines. Treating a gun as a routine matter could be unsafe, but so can treating driving as a routine matter. Both can be catastrophically deadly. Most likely the lawyer needs more training. Now the drunk guy who brought his gun to a movie theater in Renton, Washington? He is definitely irresponsible. Guns and alcohol do not mix.