AH :
Yeah, as far as I can recall, she was never able to sense the bug in XL unless he allowed her to (e.g., when he used it to call her in chapter 8).
This is certainly true in Fangfeng Bei's case, when she did not know who he really was.
When they first planted the Lovers bug, there were totally no barriers between them and Xiao Liu felt the beating of Xiang Liu's heart so clearly.
Xiao Liu tightly clasped the walnut in his palm and started chanting, summoning the bug in his own body.
After some time, Xiao Liu felt his own heart start to beat wildly, and incredibly he could feel another heart beating as well. The two hearts were like long-lost friends who were reunited, beating in response to each other. Xiao Liu hesitantly reached out his hand and put it on Xiang Liu’s chest. It was indeed Xiang Liu’s heart.
-- Vol 1 Ch7
Prior to the reveal of Fangfeng Bei's real identity, Xiao Yao could not explain her sense of familiarity over Fangfeng Bei, except that it was instinctive - and this was even before he started teaching her archery. Although logically she thought he could not be Xiang Liu, the sense of familiarity could only have been from her own body's instinctive recognition of Xiang Liu.
Every detail pointed to Fangfeng Bei not being Xiang Liu. Sons of powerful families such as Fangfeng Bei would be known by many people so there was no way for Xiang Liu to impersonate him. Yet Xiao Yao felt he was familiar even though that sense of familiarity could not be analysed rationally or put to words. It was just an instinctive feeling that her body felt.
:
She saw Fangfeng Bei on a winged horse, standing at the end of the street. It was dark so she could not clearly see who the person on the winged horse was but her instinct told her he was there.
-- Vol 1 Ch15
Throughout that time, she never got a response from the bug because Xiang Liu suppressed the bugs' reaction towards her. Based on what we know from how the bugs responded when they were planted, I can only imagine that he continued to suppress its response to her from that moment on to the end, except for times when he clearly wanted her to feel it, or when he was involuntarily affected:
The seashell was very large and even if two people laid inside, it was not cramped at all. While she was unconscious, she slept here for thirty-seven years with Xiang Liu, was that equivalent to sleeping in the same bed and sharing the same pillow? Those two merpeople saw the seashell as their love nest, how did Xiang Liu view this seashell?
Xiao Yao suddenly felt her mind racing with all kinds of thoughts, and her face burned as her heart sped up.
The realisation left her secretly dismayed. She could control her expressions and reactions, but she could not control her heartbeat. Sure enough, Xiang Liu sensed it immediately and looked over at her. Xiao Yao quickly said, “I’m hungry! So hungry that I’m flustered!”
Xiao Yao’s face was as red as the burning sunset so she tried hard to force her eyes wide open and stare at Xiang Liu. His heartbeat quickened a few times. Just as Xiao Yao sensed it, it was gone just like that and she thought she must have imagined it because she was in a panic.
-- Vol 2 Ch9 (Chapter 26)
When she crossed paths with the richly dressed man, it was at first an unplanned meeting on the streets, not somewhere he was meeting Jing, so he could not have planned for her to be there and catch sight of him. As such, it is possible that his guard was down and she was so in tune with him that she instinctively sensed his presence:
One day Xiao Yao left the medical clinic, chatting with Miao Pu as they walked.
It was almost dark and the street was filled with people rushing home to their families. For some unknown reason, Xiao Yao spotted a richly dressed man at a glance in the sea of people. She kept staring at the man but he did not even glance at her. They passed each other by and the man continued on his way straight ahead while Xiao Yao gradually stopped, turning back to look at him.
Miao Pu asked curiously, “What did my lady see?”
Xiao Yao stood there in a daze for a moment before suddenly rushing off after the man but with so many people on the street she could not find him anymore. Refusing to give up, she continued to run while searching around everywhere.
Not knowing what had happened, Miao Pu followed Xiao Yao closely, asking, “What is my lady looking for?”
“I… I… don’t know either.” Xiao Yao was not lying to Miao Pu, she genuinely did not know.
-- Vol 3 Ch5 (Chapter 38)
I guess going based on this, I would then imagine that in the drama, he deliberately suppressed the Lovers bugs, so that Xiao Yao would have no idea it was him at the time.