I had a colleague a lot like this. In fact, I thought it might have been one and the same person until I saw your reference to the type of work you do (client-led).
My colleague and I worked in an office with maybe 30 people. We weren't on the same team, but we were at the same bank of desks for about 18 months. We were both professional women in our early 30s. I liked her well enough but I couldn't say I really knew her because she was never there.
My colleague was supposed to be at her desk 8/8:30 - 6/ 6:30 Mon-Fri. Her work would've taken her out of the office from time to time, but typically only for a breakfast meeting here or an two hour meeting there. Occasionally, there would be something that would take you out for nearly the whole day. Typically, the other people, in our office and across the industry, doing a similar job would be in the office most of the time.
My colleague, however, would come into the office sometime in the morning, stay for maybe an hour. Then maybe reappear for another hour or two in the afternoon, but usually she didn't. She was always hurrying - running in clutching her briefcase, and running back out again because she was late for a meeting.
We often didn't see her for two or three days in a row. TBH we got used to her chair being empty so much so that we'd occasionally have conversations about when she was last spotted and we'd speculate that maybe she'd been on holiday or something.
Her line manager worked in a different city which is probably how she got away with it.
Someone from another company we did business with told me one day that everyone talked about her. There was a lot of speculation that she had two jobs and every time she wrote a new research report she just circulated to two companies.
What sticks in my mind though is one conversation I had with her. It was a Friday and she was at her desk all morning (unusual). She saw an email referencing a major IT system change and she asked why this was the first we were hearing of it. I told her that there had been a big presentation to the whole staff on Tuesday. She said that she was in the office on Tuesday, so how could she have missed it? I explained that everyone in the office had huddled into a conference room for an hour to hear it, and maybe she was out at that time? (I was thinking to myself that she hadn't been in at all on Monday, Tuesday or Wednesday). She started to get angry, hotly insisting that she'd been in all day Tuesday. Moreover, she was almost always in the office. Everyone else was looking at her with their jaws hanging open. I still don't know if she was lying to herself or trying to brainwash us.
In the end, she got made redundant in the sort of redundancy round where they are getting rid of people whose faces no longer fit, rather than whose job function is no longer required. So, the senior management team must have known after all.