Maybe hire a guard or something to watch over her? Considering the work Hawke did, I sincerely doubt many of the low-life scum liked him.
Speculation.
We know his only known legitimate business enterprise paid protection money to the Coterie, and they though well enough of him to take his word when he was found kneeling over the murdered body of the person they sent to assist him.
We know that Varric paid bribes to the various other underworld organizations, and he was a known associate of Hawke.
We know, and they would know, that Hawke was close friends with the Captain of the Guard, and in a similar vein that Hawke had personally opened up new opportunities for surviving underworld organizations by disposing of the competition.
Then there's the matter of not seeing any other nobility wandering around with a full guard compliment. In fact, the only 'noble' house we visit that has any guards at all outside the Viscounts palace is Bartrand's, and he was flat out insane.
Basically, Hawke acted exactly like all the other nobility, with the added bonus of being a murder machine to those that crossed him.
Then there's the fact that Liandra had been making that exact same trip for three years without mishap. Why would she need guards?
Not that it would have mattered, mind you, as shown in Act 3. Carver/Bethany could be stationed in an outpost of the Grey Wardens outside Kirkwall entirely and still be kidnapped with ease by lunatics and idiots.
No, I'm not buying that guards were a reasonable or necessary response to Hawke's situation, nor that they would have had any effect whatsoever.