Cadfael said Columbanus did not need the poppy syrup on this trip, and Cadfael thought the events of that day cast doubt on Jerome as the murderer, not Columbanus  until his realization that Jerome did not sleep on purpose.  Now Cadfael cast his doubts on Columbanus.  Cast doubts, not provided alibis.  

The brothers have no private property, so he can search the scrip any time.  Peredur's mother is a minor plot device so he does search; the main point is the discovery and Cadfael's new view of that day. 
Perhaps you could provide the text quotes for your changes, next time.

Quotes from the text of A Morbid Taste for Bones

Jerome lacks an alibi, rather than providing one for Columbanus in Chapter Six
"Father, you sent me with Brother Jerome, yesterday, to keep vigil in the chapel, and pray earnestly for a good outcome, in amity and peace. Father, we came there in good time, before eleven, as I judge, and having eaten our meal, we went in and took our places, for there are prayer-desks within, and the altar is kept clean and well-tended. Oh, Father, my will to keep vigil was good, but the flesh was weak. I had not been half an hour kneeling in prayer, when I fell asleep on my arms on the desk, to my endless shame. It is no excuse that I have slept badly and thought much since we came here. Prayer should fix and purify the mind. I slept, and our cause was weakened. I must have slept all the afternoon, for the next thing I remember is Brother Jerome shaking me by the shoulder and telling me there was a messenger calling us to go with him."

He caught his breath, and a frantic tear rolled down his cheek, circling the bold, rounded Norman bone. "Oh, do not look askance at Brother Jerome, for he surely never knew I had been sleeping, and there is no blame at all to him for not observing and reporting my sin. I awoke as he touched me, and arose and went with him. He thought me as earnest in prayer as he, and knew no wrong."
Nobody, probably, had thought of looking askance at Brother Jerome until then, but Cadfael was probably the quickest and most alert, and the only one who caught the curious expression of apprehension, fading rapidly into complacency, that passed over Brother Jerome's normally controlled countenance. Jerome had not been pursuing the same studies as Cadfael, or he would have been far from complacent. For Brother Columbanus in his self-absorbed innocence had just removed all certainty that Jerome had spent the previous noon and afternoon motionless in Saint Winifred's chapel, praying for a happy solution. His only guarantor had been fast asleep throughout. He could have sauntered out and gone anywhere he chose.

Cadfael speaking to Peredur in Chapter Nine
"Never mind, you can show me where they lodge. Columbanus brought some of my poppy syrup with him, in case of need, the phial should be there with his scrip, he'd hardly have it on him. And as far as I know, he's had no occasion to use it, his cantrips here in Wales have been of a quieter kind. We can find a use for it now."