As he sees it, Marco is (probably) the most suitable person to track Carmen and observe what she is doing. Whilst he has no sorcerous ability and limited skills in these things, he does have a permanent link to her which she has (hopefully) not considered him using against her. He has no illusions about her ability to destroy him at will but there is a chance that he will be ignored as insignificant till it's too late. He guesses that she uses the heart as a focus for empathic magics, most likely in social situations.
In order to avoid detection, he started in a passive mode by just relaxing and letting himself feel the effects of when Carmen uses his heart or when it is affected by her surroundings (eg magical devices, effects or creatures). At first this was very faint and unspecific but with experience he was able to start identifying repeating patterns and build up a very rough picture of what is happening.
With time these patterns started to be linked to likely behaviour eg first consciousness of the day, falling asleep, making love (an intense emotion prior to sleep), surprise, fear, etc. At the same time he started to get a feel for how far away she was and in which direction. Pulling all of this together gave hints on where she is (timezone from sleep cycles and distance away and then travellers knowledge to pick up things like if she's in Spain and asleep by 11 then she's not in Madrid or any of the major towns).
After some time mastering this he was ready to start tracking her movements and following her, initially just in the same country and gradually getting closer.
As cover, he built a reputation of an increasingly dissolute wanderer, randomly moving around the world (leaving a few hints that suggest he felt drawn to different places without knowing why). He knew that at some point along the way she will notice him again at which point he hopes she will play with him (rather than kill him).
Some notes from his diary :
Feb 1915, left Mexico
01-Mar : if I lie very still for a long time in this dark room I can feel Her like an elusive scent. Reluctantly ignoring the impulse to concentrate on it, I just let the sensation pass over. I think I have a chance now but mustn't rush it. If she notices me it's all over.
07-Mar : I'm starting to notice different sensations now. It's like choosing scents at first they all smell they all smell the same but with practice it becomes easier.
09-Mar : I laid here for hours today and I?m starting to see patterns. It's difficult to describe the sensation but I have come to associate them with smells. There's one recurring sharp, citrus scent which seems to happen most days. Is this when she wakes up in the morning perhaps?
16-Mar : I find I can now pick up the scents just by relaxing and being quiet, it's still mostly a confusion of different smells and textures which mean very little but I can definitely perceive them.
17-Mar : lazy, musky scent which seems to slowly fade followed by almost nothing for many hours, punctuated only by occasional vivid pulses. Sleep and dreams?
30-Mar : it seems that there are 2 distinct types of smell, one fresher, the other more sophisticated. Is this her 2 personalities?
4-Apr : seems to be stronger from the East, will try travelling towards it.
17-Apr : got stronger and stronger as I approached Paris. Then suddenly it changed and headed south. Caught the train to Nice but when I got there the scent became faint and directionless.
May 1915, was summoned by the War Council to leave for Egypt.
June 1915, during most of the trip along the Nile, Carmen seemed to be in England and very busy. She has a lover or lovers.